Does the channel feel smooth to you when you try to combo with it? It clunks it up for me.
I just want it a little shorter so his combo feels better when used.

Remember I wanted something to help his early farming a bit, before a Deathcap.

Whenever I hit six I try to get a kill as much as possible, E, easy stun proc with one R, Q, R, R, and try to Q again if they survived. If they survive, then next time I would poke them down before killing.

It feels smooth to me- ideally I W E R Q R Q R in a burst combo once I have CDR,

I don't think he really needs help farming, and would not like a buff to it if it came with a nerf of any kind.

The range on the Q with W up is probably what keeps it from feeling clunky for me: its already so easy to land it would probably be broken with a faster channel.

He doesn't need help farming at all, I agree.

I think I might try what you suggest, and go Mana/CDR runes.

The channel is "smooth" if you understand how it works. most people will mash the buttons so fast then get frustrated when they miss skill shots. Remember that you can essentially queue up spells while you are are channeling into his root. So people que up his spells then they miss because the target has moved. Try slowing down and queueing the spells just before his root finishes channeling. This gives a much smoother feel and alot more rewarding result .

As far as early farming I think he does ok. With the runes and masteries i use I typically build 2 dorans rings into a haunting guise followed by Sorc shoes. This give him a boost to his damage and enough pen to make sure the damage isnt shrugged off. especially if you are facing another ap mid. They have only ~50 armor to begin with. Shred 30 of that with just items(no runes masteries and w) and they are in for some hurt. If you miss no big deal thats why his q is on a short cool down and the double dorans helps to sustain mana to keep it up.

One big thing I can recommend too is try to couple last hitting with poke. Xerath excels at that.

Someone link to the thread about making him immune to CC while in turret mode (W).

To make it clear, because I didn't make it clear, by more base damage, something like 10 so it could finish the casters.

Though his clunkyness, to me, might be because it just felt weird to jump from the cast time of the ult to the cast time of his Q.

He isn't immune to cc. He just will not move if taunted or charmed while rooted. The spell still has its effect.

EDIT: Read that wrong. Thought you meant he was immune while rooted.

If you rush a Chalice like I do, mana isn't much of a problem no matter what your runes are, though other builds work to.

Personally, I bring Teleport so that if I'm ever getting low I can Recall and buy with no penalty; and in the right situations I can cover/ countergank another lane with it with not travel risks.

Its pretty easy to sneak an ult cast in between Qs.

You mean the thread where someone suggested that CC can't move him if he uses W? Morello did say he tried to suggest it or something to the live team but it's been months. I guess he forgot, or it was buried.